September 25, 2003
11:00 AM (EDT)

News Release Number: STScI-2003-29

Hubble Uncovers Smallest Moons Yet Seen Around Uranus

September 25, 2003: Astronomers have discovered two of the smallest moons yet found around
Uranus. The new moons, uncovered by NASA's Hubble Space Telescope, are
about 8 to 10 miles across (12 to 16 km) — about the size of San
Francisco. The two moons are so faint they eluded detection by the
Voyager 2 spacecraft, which discovered 10 small satellites when it flew
by the gas giant planet in 1986. The newly detected moons are orbiting
even closer to the planet than the five major Uranian satellites, which
are several hundred miles wide. The two new satellites are the first
inner moons of Uranus discovered from an Earth-based telescope in more
than 50 years. The International Astronomical Union (IAU) will announce
the finding today. The Hubble telescope observations also helped
astronomers confirm the discovery of another tiny moon that had
originally been spotted in Voyager pictures.

Q & A: Understanding the Discovery

1.
Are there more moons around Uranus that astronomers have not found?

Astronomers think there may be small, undetected moons orbiting close to
Uranus. The structure of Uranus's system of 10 rings suggests that more
than the 13 known inner moons are needed to keep the rings in place.

2.
Why are the newly discovered moons so faint?

The moons are small and dark, which suggests they are composed of rocky
material. The moons are so dark they don't reflect much light.

3.
Why didn't the Voyager 2 spacecraft see the moons?

Voyager, which passed within 51,000 miles from Uranus's cloud tops,
snapped relatively short-exposure pictures of the planet and its
satellites. The moons are so faint they were not detected in the
short-exposure photographs. Astronomers used the Hubble telescope's
Advanced Camera for Surveys to take long-exposure photographs (4 minutes
each) of the planet to pinpoint the dim moons.